LED lighting devices

ABSTRACT

Packaged chip-on-board (COB) LED arrays are provided where a color conversion medium is distributed within a glass containment plate, rather than silicone, to reduce the operating temperature of the color conversion medium and avoid damage while increasing light output. A lighting device is provided comprising a chip-on-board (COB) light emitting diode (LED) light source, a light source encapsulant, a distributed color conversion medium, and a glass containment plate. The COB LED light source comprises a thermal heat sink framework and at least one LED and defines a light source encapsulant cavity in which the light source encapsulant is distributed over the LED. The glass containment plate is positioned over the light source encapsulant cavity and contains the distributed color conversion medium. The light source encapsulant is distributed over the LED at a thickness that is sufficient to encapsulate the LED and define encapsulant thermal conduction paths.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional Application Ser. No. 61/731,530, filed Nov. 30, 2012 (SP12-370P) and is a continuation-in-part of U.S. patent application Ser. No. 13/281,671, filed Oct. 26, 2011 (SP10-290), which claims the benefit of U.S. Provisional Application Ser. No. 61/407,710 filed Oct. 28, 2010 (SP10-290P).

BACKGROUND

Field

The present disclosure relates to light emitting diode (LED) lighting devices and, more particularly, packaged chip-on-board (COB) LED arrays.

Technical Background

Referring initially to FIG. 1, high brightness LED lighting devices, i.e., light sources approaching or exceeding 1000 lumens, typically require a significant number of blue LEDs 10 configured in a two-dimensional array that is secured, for example, to a metal clad PC board 20. In many cases, the diode array is covered by a color conversion phosphor dispersed in a silicone encapsulant 30. These and other types of COB LED arrays are becoming standardized in shape, light output, and electrical drive requirements and could conceivably become the new lighting standard.

BRIEF SUMMARY

The present inventors have recognized that a significant metric for packaged chip-on-board (COB) LED arrays is light output, measured in lumens per LED, with the understood objective of maximizing light output per LED while minimizing cost per LED. Light output per LED is, however, limited by the temperature rise of the phosphor and the impact of that rise on the surrounding silicone. Due to the inherent conversion inefficiency of the phosphor as well as Stokes shift during color conversion, some of the blue light is converted into heat, which can be removed by thermal conduction through the LED to an underlying heat sink. Unfortunately, the silicone potting compound in which the phosphor is mixed has a relatively low thermal conductivity—a condition that can cause a significant temperature rise in the phosphor-in-silicone film. For example, given a heat sink temperature of 85° C.@ 1000 lumens, the temperature of the phosphor-in-silicone film can reach 160 degrees, which is the maximum operating temperature of the silicone but typically does not correspond to the maximum light output or temperature of the LED. Accordingly, the present disclosure introduces means by which heat can be more efficiently removed from the color converting layer of an LED lighting device to allow the LED(s) of the device to be driven harder, increasing total light output.

For example, in chip-on-board (COB) LED arrays, blue LEDs are often encapsulated in what starts out as a slurry of phosphor and silicone. The thickness of the phosphor-in-silicone (PiS) above the LEDs has been measured at 750 μm. This is sufficient to convert a portion of the blue light to longer wavelengths while allowing some of the blue light to pass through unconverted. As the blue light is converted by the phosphor, some heating occurs due to quantum efficiency being less than perfect, e.g., about 95%. Additional heating occurs due to Stokes shift as a higher energy blue photon is traded for a lower energy photon of longer wavelength. Since silicone is a relatively poor thermal conductor, this heat turns out to limit the output of the blue LEDs. That is, if the blue LEDs were driven harder, then the PiS would heat to the point that the silicone would become damaged.

According to the subject matter of the present disclosure, packaged chip-on-board (COB) LED arrays are provided where a color conversion medium is distributed within a glass containment plate, rather than silicone, to reduce the operating temperature of the color conversion medium and avoid damage while increasing light output. The glass containment plate may be provided as a glass containment frame comprising an interior volume for containing a color conversion medium, a glass containment matrix in which the color conversion is distributed, or any other substantially planar structural glass member, vessel, or assembly suitable for containing the color conversion medium.

The glass containment plate, loaded with the color conversion medium, sits just above the wire bonds of the LED array. Plain silicone can be used to surround the LEDs, rather than air, which is a poor thermal conductor. This means that the thickness of the silicone above the LEDs can be reduced to the height of the wire bonds, i.e., about 50 μm with the very low profile variety of wire bonds. This structure is beneficial in a number of ways. First the color conversion medium can itself withstand higher temperature than cases where the medium is dispersed in silicone because the glass containment plate has no organic component. Further, the ability to channel heat to the heat sink of the packaged LED is greatly improved because the thickness of the silicone layer above the LEDs is greatly reduced, e.g., from about 750 μm to about 50 μm. It is contemplated that even further thickness reduction would be possible with a flip-chipped LED array as wire bonds would no longer set the thickness requirement. Although perhaps initially counterintuitive, the heat dissipation path for the heat generated by the phosphor is through the LEDs themselves.

The glass containment plate of the present disclosure is also beneficial because it provides for additional manufacturing process control. Specifically, the plate can be tested separately from the corresponding LED array and an appropriate plate-to-array pairing can be made to achieve the desired color output. This is not the case when a conversion medium is provided as a slurry in the silicone used to encapsulate the LED array.

In accordance with one embodiment of the present disclosure, a lighting device is provided comprising a chip-on-board (COB) light emitting diode (LED) light source, a light source encapsulant, a distributed color conversion medium, and a glass containment plate. The COB LED light source comprises a thermal heat sink framework and at least one LED and defines a light source encapsulant cavity in which the light source encapsulant is distributed over the LED. The glass containment plate is positioned over the light source encapsulant cavity and contains the distributed color conversion medium. The light source encapsulant is distributed over the LED at a thickness that is sufficient to encapsulate the LED and define encapsulant thermal conduction paths T_(PE) (see FIG. 2) extending through the light source encapsulant to the thermal heat sink framework from the distributed color conversion medium.

In accordance with another embodiment of the present disclosure, the glass containment plate comprises a glass matrix and the distributed color conversion medium comprises a phosphor distributed in the glass matrix. In accordance with yet another embodiment of the present disclosure, the distributed color conversion medium comprises a quantum dot structure contained within an interior volume of the glass containment plate. In accordance with a further embodiment of the present disclosure, the distributed color conversion medium comprises a phosphor distributed in a glass matrix and the lighting device further comprises a quantum dot plate disposed over the glass containment plate to define a supplemental emission field of the lighting device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

FIG. 1 illustrates an LED lighting device employing a phosphor-in-silicone color conversion medium;

FIG. 2 is a schematic illustration of an LED lighting device according to one embodiment of the present disclosure;

FIG. 3 is a schematic illustration of an LED lighting device according to another embodiment of the present disclosure; and

FIG. 4 is a schematic illustration of an LED lighting device according to yet another embodiment of the present disclosure.

DETAILED DESCRIPTION

FIGS. 2-4 illustrate COB LED lighting devices 100, 100′, 100″ that comprise at least one LED 110, a light source encapsulant 120, a distributed color conversion medium 130, a glass containment plate 140, 140′, and a thermal heat sink framework 150 in the form of, for example, a metal clad printed circuit board. The color conversion medium 130, is distributed in two dimensions over an emission field of the LED lighting device within the glass containment plate 140, 140′ and may comprise, for example, a color converting phosphor or a quantum dot structure. Beyond that which is disclosed herein, the specific materials selected for the light source encapsulant 120, color conversion medium 130, glass containment plate 140, 140′ and the thermal heat sink framework 150 can be gleaned from references like US PG Pub. No. 2012/0107622, which relates primarily to the use of color converting phosphors in LED lighting devices, US 2012/0175588, which relates to the use of light-converting, colloidal, doped semiconductor nanocrystals to provide monochromatic and white light sources based on LEDs, and U.S. Pat. No. 7,723,744, which relates to light-emitting devices that incorporate one or more underlying LED chips or other light sources and a layer having one or more populations of nanoparticles disposed over the light source. The nanoparticles absorb some light emitted by the underlying source, and re-emit light at a different level. By varying the type and relative concentration of nanoparticles, different emission spectra may be achieved.

The COB LED light source 100, 100′, 100″ defines a light source encapsulant cavity in which the light source encapsulant 120 is distributed over the array of LEDs 110. The glass containment plate 140, 140′ is positioned over the light source encapsulant cavity, contains the distributed color conversion medium 130, and defines glassy thermal conduction paths T_(PG) extending through the glass containment plate 140, 140′ to the thermal heat sink framework 150 from the distributed color conversion medium 130. Further, the light source encapsulant 120 is distributed over the array of LEDs 110 at a thickness that is sufficient to encapsulate the LEDs 110, including their wire bonds and any other LED hardware, and define encapsulant thermal conduction paths T_(PE) extending through the light source encapsulant 120 to the thermal heat sink framework 150 from the distributed color conversion medium 130.

As is noted above, the present disclosure introduces means by which heat can be more efficiently removed from the color converting layer of an LED lighting device and means that allow for a greater absolute temperature rise in the color converting layer. Both of these factors allow the LED(s) of the device to be driven harder, increasing total light output. To this end, the thickness of the light source encapsulant 120 is preferably tailored such that the thermal conduction paths T_(PE) extend less than approximately 100 μm through the light source encapsulant 120. More preferably, it is contemplated that the thickness of the light source encapsulant can be tailored such that the thermal conduction paths T_(PE) extend less than approximately 50 μm through the light source encapsulant 120.

The thermal performance of the structure may be expressed in terms of the thermal resistances of the heat paths T_(PG) and T_(PE), both of which are illustrated schematically in FIGS. 2-4. For practical dimensions, the relatively vertical heat path T_(PE) dominates, mostly because its path is shorter than that of T_(PG). Additionally, the thermal resistance of the traditional phosphor-in -silicone LED structure of FIG. 1 is approximately 5 times greater than the phosphor-in-glass design illustrated in FIG. 2. The relatively thin glass encapsulant matrix design of FIG. 2 reduces the temperature rise of the phosphor fivefold at comparable LED powers, which enables the LEDs to be driven at higher currents to produce more light. This advantage stems predominantly from the thin profile of the glass containment plate 140 and the reduced thickness of the encapsulant layer 120 above the LEDs 110. In terms of thermal resistance, it is contemplated that the thickness of the light source encapsulant 120 can be tailored such that the thermal conduction paths T_(PE) encounter a thermal resistance of less than approximately ⅕ that of a traditional phosphor in silicone package, such as that depicted in FIG. 1. In one embodiment, this thermal resistance is less than approximately 15° C./W through the light source encapsulant 120.

In the configurations of FIGS. 2 and 3, the glass containment plate 140 comprises a glass matrix and the distributed color conversion medium 130 comprises a phosphor distributed in the glass matrix, as disclosed in US PG Pub 2012/0107622 A1. In the embodiment of FIG. 2, the LED lighting device 100 further comprises a glass cover plate 145 disposed over the glass matrix, with ion exchanged glass being a suitable contemplated glass composition choice for the glass cover plate 145. The glass containment plate 140 can be permanently bonded to the glass cover plate 145 during firing of the two, to consolidate the frit of the glass containment plate 140. In many embodiments, particularly where the glass containment plate 140 is provided as a glass containment matrix in which the color conversion medium 130 is distributed, it will be advantageous to provide the glass containment plate 140 by tape casting the material of the glass containment plate to a glass substrate and then bonding that substrate to the cover glass plate 145, which occurs during consolidation of the frit. However, it is contemplated that the material of the glass containment plate 140 may be tape cast directly onto the cover glass plate 145, thus avoiding the need to bond the glass containment plate 140 to the cover glass plate 145.

In the embodiment of FIG. 3, the LED lighting device 100′ further comprises a quantum dot plate 160 disposed over the glass containment plate 140 to define a supplemental emission field of the LED lighting device 100′. The quantum dot plate 160 comprises a quantum dot structure 170 that is contained within an interior volume defined between opposing, sealed glass panels 160 a, 160 b of the quantum dot plate 160. The primary emission field that is defined by the distributed phosphor color conversion medium 130 is spatially congruent with, but spectrally distinct from, the supplemental emission field defined by the quantum dot plate 160. In this manner, the emission spectrum of the emission field defined by the quantum dot plate 160 can be tailored to add optical warmth to the emission spectrum of the emission field defined by the distributed phosphor color conversion medium 130. For example, where the distributed phosphor color conversion medium 130 converts blue light from the LEDs 110 to yellow, the quantum dots of the quantum dot plate can be tailored to add warmth by converting some of the yellow light, as well as leaking blue light, to red—one advantage being that red quantum dots have a relatively narrow emission band, unlike red phosphors which waste light by tailing into the IR. In the case of red quantum dots, since quantum dots have a relatively narrow emission band, the issue of tailing into the IR can be avoided thus preserving good power efficiency. As an alternative to selecting a quantum dot plate of a particular color, it is contemplated that the sizes of the quantum dots contained can be adjusted to obtain the desired color. It is also contemplated that a variety of quantum dot sizes can also be blended to obtain a particular color, e.g., white.

In the embodiment of FIG. 3, and similar embodiments, the glass containment plate 140 and the quantum dot plate 160 must be made separately because the process of tape casting and firing the glass containment plate onto the quantum dot plate 160 would damage the quantum dot structure. For this reason, a separate thin silicone bond 135 is required between the quantum dot plate 160 and the glass containment plate 140.

Referring specifically to the configuration of FIG. 4, it is noted that the glass containment plate 140′ is presented in the form of a glass containment frame comprising an interior volume defined between opposing, sealed glass panels 140 a, 140 b for containing the distributed color conversion medium 170. The distributed color conversion medium 170 may be provided in the form of the quantum dot structure described above with reference to FIG. 3. More specifically, it is contemplated that the distributed color conversion medium 170 may comprise a quantum dot structure contained within an interior volume defined by opposing glass panels 140 a, 140 b, with flexible fusion glass being a suitable contemplated glass composition choice. In FIGS. 3 and 4, the cover glass plate 145 of FIG. 2 is eliminated because the glass containment plates 140′,160′, i.e., the quantum dot plates, can serve as the protective cover glass.

In the quantum dot structure illustrated in FIGS. 3 and 4, the opposing, sealed glass panels comprise one cavity glass 140 a, 160 a and one sealing glass 140 b, 160 b. The sealing glass 140 b, 160 b is typically a relatively thin (about 100 μm) display grade glass, such as Willow which is a very thin (typically 100 μm) version of EAGLE XG® display glass available from Corning, Incorporated. A suitable cavity can be provided in the cavity glass 140 a, 160 a by any conventional or yet to be developed glass molding or glass machining technique including, for example, micromachining, laser-assisted machining or milling, laser ablation, etching, or combinations thereof. Sputtered glass can then be deposited on the underside of the sealing glass 140 b, 160 b and a laser can be used to peripherally bond the sealing glass 140 b, 160 b to the cavity glass while the quantum dots are resting in the cavity.

According to one set of contemplated embodiments, sealed glass panels for containing the aforementioned quantum dots may be constructed by providing a relatively low melting temperature (i.e., low T_(g)) glass sealing strip along a peripheral portion of a sealing surface of the sealing glass, the cavity glass, or both. In this manner, the cavity glass and the sealing glass, when brought into a mating configuration, cooperate with the glass sealing strip to define an interior volume that contains the quantum dots. The glass sealing strip may be deposited via physical vapor deposition, for example, by sputtering from a sputtering target.

A focused laser beam can be used to locally melt the low melting temperature glass sealing strip adjacent glass substrate material to form a sealed interface. In one approach, the laser can be focused through either the cavity glass or the sealing glass and then positionally scanned to locally heat the glass sealing strip and adjacent portions of the cavity glass and sealing glass. In order to affect local melting of the glass sealing strip, the glass sealing strip is preferably at least about 15% absorbing at the laser processing wavelength. The cavity glass and the sealing glass are typically transparent (e.g., at least 50%, 70%, 80% or 90% transparent) at the laser processing wavelength.

In an alternate embodiment, in lieu of forming a patterned glass sealing strip, a blanket layer of sealing (low melting temperature) glass can be formed over substantially all of a surface of sealing glass. An assembled structure comprising the cavity glass/sealing glass layer/sealing glass can be assembled as above, and a laser can be used to locally-define the sealing interface between the two substrates.

Laser 500 can have any suitable output to affect sealing. An example laser is a UV laser such as a 355 nm laser, which lies in the range of transparency for common display glasses. A suitable laser power can range from about 5 W to about 6.15 W.A translation rate of the laser (i.e., sealing rate) can range from about 1 mm/sec to 100 mm/sec, such as 1, 2, 5, 10, 20, 50 or 100 mm/sec. The laser spot size (diameter) can be about 0.5 to 1 mm.

The width of the sealed region, which can be proportional to the laser spot size, can be about 0.1 to 2 mm, e.g., 0.1, 0.2, 0.5, 1, 1.5 or 2 mm. A total thickness of a glass sealing layer can range from about 100 nm to 10 microns. In various embodiments, a thickness of the layer can be less than 10 microns, e.g., less than 10, 5, 2, 1, 0.5, or 0.2 microns. Example glass sealing layer thicknesses include 0.1, 0.2, 0.5, 1, 2, 5 or 10 microns.

In various embodiments of the present disclosure, the material of the glass sealing strip is transparent and/or translucent, relatively thin, impermeable, “green,” and configured to form hermetic seals at low temperatures and with sufficient seal strength to accommodate large differences in CTE between the sealing material and the adjacent glass substrates. Further, it may be preferable to ensure that the material of the sealing strip is free of fillers, binders, and/or organic additives. The low melting temperature glass materials used to form the sealing material may or may not be formed from glass powders or ground glass.

In general, suitable sealing materials include low T_(g) glasses and suitably reactive oxides of copper or tin. The glass sealing material can be formed from low T_(g) materials such as phosphate glasses, borate glasses, tellurite glasses and chalcogenide glasses. As defined herein, a low T_(g) glass material has a glass transition temperature of less than 400° C., e.g., less than 350° C., 300° C., 250° C., or 200° C. Example borate and phosphate glasses include tin phosphates, tin fluorophosphates, and tin fluoroborates. Sputtering targets can include such glass materials or, alternatively, precursors thereof. Example copper and tin oxides are CuO and SnO, which can be formed from sputtering targets comprising pressed powders of these materials.

Optionally, glass sealing compositions can include one or more dopants, including but not limited to tungsten, cerium and niobium. Such dopants, if included, can affect, for example, the optical properties of the glass layer, and can be used to control the absorption by the glass layer of laser radiation. For instance, doping with ceria can increase the absorption by a low T_(g) glass barrier at laser processing wavelengths.

Example tin fluorophosphate glass compositions can be expressed in terms of the respective compositions of SnO, SnF₂ and P₂O₅ in a corresponding ternary phase diagram. Suitable tin fluorophosphates glasses include 20-100 mol % SnO, 0-50 mol % SnF₂ and 0-30 mol % P₂O₅. These tin fluorophosphates glass compositions can optionally include 0-10 mol % WO₃, 0-10 mol % CeO₂ and/or 0-5 mol % Nb₂O₅.

For example, a composition of a doped tin fluorophosphate starting material suitable for forming a glass sealing layer comprises 35 to 50 mole percent SnO, 30 to 40 mole percent SnF₂, 15 to 25 mole percent P₂O₅, and 1.5 to 3 mole percent of a dopant oxide such as WO₃, CeO₂ and/or Nb₂O₅.

A tin fluorophosphate glass composition according to one particular embodiment is a niobium-doped tin oxide/tin fluorophosphate/phosphorus pentoxide glass comprising about 38.7 mol % SnO, 39.6 mol % SnF₂, 19.9 mol % P₂O₅ and 1.8 mol % Nb₂O₅. Sputtering targets that can be used to form such a glass layer may include, expressed in terms of atomic mole percent, 23.04% Sn, 15.36% F, 12.16% P, 48.38% O and 1.06% Nb.

A tin phosphate glass composition according to an alternate embodiment comprises about 27% Sn, 13% P and 60% O, which can be derived from a sputtering target comprising, in atomic mole percent, about 27% Sn, 13% P and 60% O . As will be appreciated, the various glass compositions disclosed herein may refer to the composition of the deposited layer or to the composition of the source sputtering target.

As with the tin fluorophosphates glass compositions, example tin fluoroborate glass compositions can be expressed in terms of the respective ternary phase diagram compositions of SnO, SnF₂ and B₂O₃. Suitable tin fluoroborate glass compositions include 20-100 mol % SnO, 0-50 mol % SnF₂ and 0-30 mol % B₂O₃. These tin fluoroborate glass compositions can optionally include 0-10 mol % WO₃, 0-10 mol % CeO₂ and/or 0-5 mol % Nb₂O₅.

Additional aspects of suitable low T_(g) glass compositions and methods used to form glass sealing layers from these materials are disclosed in commonly-assigned U.S. Pat. No. 5,089,446 and U.S. patent application Ser. Nos. 11/207,691, 11/544,262, 11/820,855, 12/072,784, 12/362,063, 12/763,541 and 12/879,578.

Regarding the thermal conduction paths T_(PE), T_(PG), it is noted that heat flow H (watts) is proportional to the associated temperature gradient, which in one dimension x is dT/dx. Mathematically

$\begin{matrix} {H = {{kA}\;\frac{d\; T}{d\; x}}} & (1) \end{matrix}$ where k is the thermal conductivity of the material and A is the cross-sectional area of an infinitesimal slab of thickness dx through which the heat flows. If the heat flow is confined to one dimension in an insulated thermal path, then the solution to equation 1 is simply

$\begin{matrix} {{{\Delta\; T} \equiv {T_{2} - T_{1}}} = {\frac{HL}{k\; A} = {R_{th}H}}} & (2) \end{matrix}$ where R_(th) is defined as the thermal resistance and L is the length of the thermal path.

For LED lighting device configurations like that illustrated in FIG. 1, the heat flow in the COB array is vertical from the phosphor through the thin (˜5 μm thick) GaN LED and the underlying sapphire substrate to the heat sink. The array can be modeled as a one-dimensional heat flow and calculate the thermal resistance using equation (2) above. Working under the assumption that a 1000 lumen array will require about 10 watts electrical input, of which about 5 watts is dissipated as heat in the LED, the remaining 5 watts is emitted as blue light. In the color conversion process, about 1.3 watts is lost as heat in the phosphor, leaving about 3.7 watts total light output. The hottest plane in the package is the surface of the phosphor. The array can be modeled as two thermal resistances in series, i.e., the phosphor-in-silicone as the first thermal resistance and the sapphire LED substrate as the second thermal resistance. The GaN film is so thin, that its thermal resistance is negligible.

Relevant specifications for the thermal model are shown in the following table:

Forward Voltage 12.2 volts Operating Current 1050 mA Junction-to-Case Thermal Resistance 0.7 deg/Watt LED lateral dimensions 1.5 mm × 1.5 mm LED thickness 0.125 mm Phosphor layer thickness (above LED) 0.757 mm Total die area 9 × (1.5 mm)² = 36 mm²

Since the thermal conductivity of sapphire is 17.35 watts/m-K at 70 degrees C., the thermal resistance (equation (2)) of the 36 mm² area, 0.125 mm thick sapphire is R_(s)=0.2 degrees/watt. The temperature rise in the phosphor layer is more complicated since the heat load is distributed throughout the film. Blue light would be expected to decay exponentially according to Beer's Law due to absorption and scatter, so the associated heat load should have the same distribution. Assuming 90% is absorbed in the t=0.757 mm thick phosphor layer, the absorption depth d, is about 0.3285 mm. The temperature of the hottest plane can be estimated assuming that the entire 1.3 watts generated in the phosphor flows through an equivalent thickness given by

$\begin{matrix} {t_{eq} = {d - \frac{t\; e^{{- t}/d}}{1 - e^{{- t}/d}}}} & (3) \end{matrix}$ with t=0.757 mm and d=0.3285 mm, the equivalent thickness t_(eq)=0.244 mm. Assuming that the thermal conductivity of the phosphor-in-silicone is 0.22 watts/m-K, the same as silicone, then the thermal resistance of the phosphor layer is R_(p)=30.8 degrees/watt, about 60 times larger than the thermal resistance of the sapphire.

Using these data, we can estimate the temperature rise of the GaN LED and the phosphor film. Given an electrical input power of 12.8 W (12.2 volts×1.05 amps), we have 8.1 watts flowing through the sapphire and 1.66 watts dissipated in the phosphor. Assuming the heat sink temperature is 85° C., the temperatures of the LED and phosphor planes would be 87° C. and 138° C., respectively and can be readily compared with similarly modeled data representing the LED lighting device of FIG. 2, where the sapphire thermal resistance is the same, 0.20 degrees/watt, and the 1.66 watts generated in the ˜150 μm thick phosphor-in-glass (PiG) film flows through the thin silicone film. The film should be as thin as possible to minimize the thermal resistance for heat flow from the phosphor-in-glass (PiG) film to the GaN heat sink and it is contemplated that a 50 μm m thickness should be enough to clear the LED wirebonds. Using the silicone thermal conductivity of 0.22 degree/m-watt, and the same 36 mm² total LED area, the thermal resistance of the vertical path is 6.3 degrees/watt.

The temperature rise of the phosphor with the same 1.66 watts thermal dissipation in the phosphor-in-glass (PiG) film is therefore 10 degrees, showing that the FIG. 2 configuration greatly reduces the phosphor temperature, as compared to the configuration of FIG. 1. These results are summarized in the following table:

Heat Sink LED Phosphor Temperature Temperature Temperature FIG. 1 85° C. 87° C. 138° C. FIG. 2 85° C. 87° C.  97° C. Similar results would be expected for the lighting device configurations of FIGS. 3 and 4.

Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed herein should not be taken to imply that these details relate to elements that are essential components of the various embodiments described herein, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Rather, the claims appended hereto should be taken as the sole representation of the breadth of the present disclosure and the corresponding scope of the various inventions described herein. Further, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects.

It is noted that recitations herein of a component of the present disclosure being “configured” in a particular way, to embody a particular property, or to function in a particular manner, are structural recitations, as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component. It is also noted that recitations herein of “at least one” component, element, etc., should not be used to create an inference that the alternative use of the articles “a” or “an” should be limited to a single component, element, etc.

It is noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure.

For the purposes of describing and defining the present invention it is noted that the terms “about” and “approximately” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.” 

What is claimed is:
 1. A lighting device comprising a chip-on-board (COB) light emitting diode (LED) light source, a light source encapsulant, a distributed color conversion medium, and a planar glass plate, wherein: the COB LED light source comprises a thermal heat sink framework and at least one LED and defines a light source encapsulant cavity in which the light source encapsulant is distributed over the LED; the planar glass plate comprises a first planar glass surface and an opposing second planar glass surface, the first planar glass surface contacting the light source encapsulant cavity; the light source encapsulant is distributed over the LED at a thickness that is sufficient to encapsulate the LED and define encapsulant thermal conduction paths T_(PE) and T_(PG) extending through the light source encapsulant to the thermal heat sink framework from the planar glass plate; the thermal conduction paths T_(PE) extend less than approximately 100 μm through the light source encapsulant and encounter a thermal resistance of less than approximately 15° C./W through the light source encapsulant; and the color conversion medium is distributed within the planar glass plate in two dimensions over an emission field of the lighting device.
 2. The lighting device as claimed in claim 1 wherein the thickness of the light source encapsulant is such that the thermal conduction paths T_(PE) extend less than approximately 50 μm through the light source encapsulant.
 3. The lighting device as claimed in claim 1 wherein the distributed color conversion medium comprises a color converting phosphor.
 4. The lighting device as claimed in claim 1 wherein the distributed color conversion medium comprises a quantum dot structure.
 5. The lighting device as claimed in claim 1 wherein: the planar glass plate comprises a glass matrix; and the distributed color conversion medium comprises a phosphor distributed in the glass matrix.
 6. The lighting device as claimed in claim 1 wherein the planar glass plate comprises a variety of quantum dot sizes that are blended to obtain a particular color.
 7. The lighting device as claimed in claim 1 wherein: the planar glass plate comprises a glass frame; and the distributed color conversion medium comprises a quantum dot structure contained within an interior volume of the glass frame.
 8. The lighting device as claimed in claim 1 wherein: the distributed color conversion medium comprises a quantum dot structure; the planar glass plate comprises opposing glass panels that are sealed at complementary edges to define an interior volume; and the quantum dot structure is contained within the interior volume of the planar glass plate.
 9. The lighting device as claimed in claim 1 wherein: the planar glass plate comprises a glass matrix; the distributed color conversion medium comprises a phosphor distributed in the glass matrix; the lighting device further comprises a quantum dot plate disposed over the planar glass plate to define a supplemental emission field of the lighting device; and the emission field defined by the distributed phosphor color conversion medium is spatially congruent with, but spectrally distinct from, the supplemental emission field defined by the quantum dot plate.
 10. The lighting device as claimed in claim 9 wherein an intervening glass plate is disposed between the glass matrix of the planar glass plate and the quantum dot plate.
 11. The lighting device as claimed in claim 9 wherein: the quantum dot plate that is disposed over the planar glass plate comprises a quantum dot structure and opposing glass panels that are sealed at complementary edges to define an interior volume; and the quantum dot structure is contained within the interior volume of the quantum dot plate.
 12. The lighting device as claimed in claim 9 wherein an emission spectrum of the emission field defined by the quantum dot plate adds optical warmth to an emission spectrum of the emission field defined by the distributed phosphor color conversion medium.
 13. The lighting device as claimed in claim 1 wherein: the COB LED light source comprises an LED array; and the light source encapsulant is distributed over the LED array.
 14. The lighting device as claimed in claim 1 wherein the color conversion medium is substantially free of organic components.
 15. The lighting device as claimed in claim 1 wherein the planar glass plate is substantially free of organic components.
 16. The lighting device as claimed in claim 1 wherein the light source encapsulant is substantially free of color conversion medium.
 17. The lighting device as claimed in claim 1 wherein the light source encapsulant cavity is substantially free of color conversion medium.
 18. A lighting device comprising a chip-on-board (COB) light emitting diode (LED) light source, a light source encapsulant, a distributed color conversion medium, and a planar glass plate, wherein: the COB LED light source comprises a thermal heat sink framework and at least one LED and defines a light source encapsulant cavity in which the light source encapsulant is distributed over the LED; the planar glass plate comprises a first planar glass surface and an opposing second planar glass surface, the first planar glass surface facing an upper light-emitting surface of the at least one LED; the light source encapsulant is distributed over the LED at a thickness that is sufficient to encapsulate the LED and define encapsulant thermal conduction paths T_(PE) and T_(PG) extending through the light source encapsulant to the thermal heat sink framework from the planar glass plate; the thermal conduction paths T_(PE) extend less than approximately 100 μm through the light source encapsulant and encounter a thermal resistance of less than approximately 15° C./W through the light source encapsulant; and the color conversion medium is distributed within the planar glass plate in two dimensions over an emission field of the lighting device. 